Core analysis and apparatus therefor



Dec. 19, 1950 w. D. ROSE coax ANALYSIS AND upm'rus max 2 Shouts-Shoot 1 Filed June 14. 1947 Fun: \uLeT F1 e.i

AVAVAVAVAVAVAVAVAVAVAVAVAV w'albcr Dean. [2056 Unventor 63 (9-7 Cl/uborheg Dec. 19, 1950 w. D.- ROSE 1 CORE mmsxs m: uvm'rus mass-on Filed Jun. 14, 1947 2 Shoots-Sheet 2 HIE-1t er Dean. Rose Unvenoor annex-neg Patented Dec. 19, 1950 CORE SIS AND APPARATUS THEREFOR Walter Dean Rose, Tulsa, Okla., assignor to Standard Oil Development Company, a corporation of Delaware Application June 14, 1947, Serial No. 754,736

4 Claims. (Cl. 73-38? The present invention is directed to a method and apparatus for the study of the static equilibrium fluid phase distribution conditions attained in subsurface formation rock when a fluid phase saturating the rock is displaced by another immiscible fluid phase.

In th estimation of oil reserves in subsurface formations and in the prediction of the producing characteristics of petroleum reservoirs information regarding the distribution of liquids in the reservoir rock and the manner in which immiscible liquids displace each other in such rock under varying conditions of temperature and pressure is extremely important. Usually this information cannot be obtained directly by analysis of reservoir rock samples because of the virtual impossibility of recovering reservoir rock samples which have not been contaminated by extraneous fluids introduced into the borehole penetrating the reservoir.

Well known and adequately described in the literature is evidence to support the theoretical equivalence of displacement experiments to displacement occurring in the natural reservoir in yielding analogous displaced and displacing phase saturation distributions when displacement equilibrium has been attained. In the apparatus hitherto employed for these experiments and the procedure followed with this apparatus it has not been possible adequately to reproduce certain reservoir conditions, particularly reservoir pressure and temperature.

The principal object of the present invention is the provision of a method and apparatus for studies of the aforesaid character in which the reservoir rock samples are placed in an environment closely approximately their natural environment in the reservoir.

' More specifically, it is an object of the present invention to provide a method and apparatus for studies of the aforesaid character in which the reservoir rock samples under examination are placed in contact with a continuous fluid phase, the displacement of which from the rock samples is to be studied, while being maintained under the pressure obtaining in the reservoirfrom which the samples were obtained and at reservoir temperature. Experimental evidence has been accumulated to indicate that laboratory displacement data give a measure of reservoir fluid phase distribution which is nearly perfect when all the An additional object of the present invention is the provision of a method and apparatus for studies of the aforesaid character in which not only the reservoir conditions of pressure and temperature may be reproduced but in which also the capillary pressure (which is the pressure difference existing across a curved interface existing between the immiscible fluid phases) may be accurately adjusted throughout the ranges normally existing in the reservoir from which the rock samples are obtained.

A particular object of the present invention is the provision in an apparatus for studies of the aforesaid character of a semi-permeable barrier in the form of a membrane composed of cellophane or cellophane-like material which, when saturated with water. is capable of resisting breakthrough by oil at high pressures.

Another specific object of the present invention is theprovision of a method and apparatus for studies of the aforesaid character in which a plurality of rock samples (in the form of plugged cores or rotary cuttings) can be processed simultaneously. This feature is of importance for many reasons. It is self-evident that the smaller the rock sample the less time will be required under otherwise similar conditions to displace a given fluid phase from the sample. However, the minimum size of an individual sample which may be studied is limited by the fact that enough fluid phase must be displaced for accurate measurement thereof. By providing an apparatus in which a large number of rock samples may be handled simultaneously the individual samples may be small enough to secure the aforesaid time advantage and at the same time the aggregate of the samples produce sufflcient displaced phase for accurate measurement. Again, this feature makes it possible to reproduce more accurately actual reservoir fluid distribution by using rock samples from differentpoints ina reservoir whereby an average value of phase displacement typical of the reservoir itself is obtained. The importance of this average value can be appreciated from the known fact that the reservoir formations are usually heterogeneous which means that at different points the reservoir rock has different Fig. 2 is an enlarged detail of Fig. 1 between the points A and A; and, i

Fig. 3 is a vertical section of an alternative embodiment of the present invention.

Referring to Fig. 1 in detail, numeral l designates a base member made of thermally conductive material of high strength, such as stainless steel, brass, or the like. Itmay be pointed out here that all of the metal parts of this apparatus are preferably composed of the same metal. Member I is a cup shaped member having on one side a drain opening 2, and at another point a threaded opening 3 to receive a nipple 4 hereinafter referred to. The upper edge of the cupshaped member is provided with an annular recess 5 carrying packing material 6 which is more properly described as a self-sealing type gasket. A metal lid 1 rests on the upper edge of the cup member and is held securely thereto by clamping ring 8. At the upper end of the lid member is an opening 9 for the introduction of fluid.

Resting on the bottom of the cup member is a metal plate I0 having a radial passage ll, opening on the upper surface of the plate at its center and extending to the periphery of the plate. Resting on the upper surface of the plate i0 is a screen [2 covered by a membrane l3. This membrane may be termed a semi-permeable membrane and is characterized by being substantially impermeable to a displacing fluid phase when it is saturated with and wetted by a fluid immiscible with said displacing fluid. Thus if saturated with and wetted by'water it is substantially impermeable to oil. An excellent material for use as this membrance is cellophane of the type commercially available. Other materials which may be used are graded collodion membranes of suitable permeability, animal membranes, porous metallic septa, and porous rubber. In practice the membrane is selected to suit the pressure at which the operation is conducted. The higher the capillary pressure which must be maintained in the operation the finer must be the capillary passages in the membrane. Thus, for higher pressure cellophane is best suited, whereas for lower pressures a more permeable material is preferred in order to cut down the time of operation.

The membrane is of somewhat larger diameter than the screen l2 and the overlapping portion of the membrane is clamped to the plate ID by a ring l4 secured by screws l5.

Screwed into the plate l0 near its periphery are two vertical posts 16, which extend to the top of the chamber formed by the lid 1. At their upper ends these posts carry a bar I! having a central opening l8 in which fits a sleeve I3 having at its upper end a flanged head 20 which rests on the bar and at its lower end an internally threaded portion 2|. In engagement with this threaded portion is a threaded rod 22 carrying at its lower end a fiat disk 23. A spring24 is held in compression between the disk and the under side of the bar.

A small tube 26 extends laterally from the plate [0 in alignment with the radial passage II and through an opening in nipple 4. Near the outer end of this tube the central passage of nipple 4 is enlarged, and in the annular space between it and the end of the tube is arranged a gasket 21. Pressing against this gasket is a sleeve 28, the outer diameter of which forms a tight friction fit with the enlarged central passage in nipple 4. The outer end of nipple 4 has a flange 29 on which is seated. the inwardly flanged end of a cap 30, which is internally threaded at its other end. In threaded engagement with this cap is a fitting 3| with which the sleeve 23 is integral.

This fitting. has a central passage 32 which terminates in an upwardly disposed cup member 33, which is part of the fitting 3|. Screwed into the upper end of the cup member is a nut 34 which presses down against packing 35 in the bottom of the cup member and holds this packing in engagement with a glass tube 36 which rests on the bottom of the cup member and passes through the nut 34. The tube is graduated for volume measurement.

The provision of cap 30 in threaded engagement with fitting 3i and adapted to engage with flange 29 on nipple 4 permits that portion of the assembly to the left of the juncture of cap 30 and flange 29, which portion of the assembly may be referred to as the cell, to be inverted for certain tests described hereinafter, while permitting tube 36 and its associated'parts to remain upright.

In the use of this device a suitably shaped rock fragment 3'! is placed on the membrane so as to be in capillary contact therewith. Let it be assumed that the object is to determine the rate and extent of displacement of water from the rock fragment by oil. The system below the membrane is filled with water and the membrane itself is saturated with water. The rock fragment is also initially saturated with water.

With the parts in the position shown, oil is admitted through the inlet port 9 through a suitable valve until reservoir conditions of capillary pressure are attained in the chamber formed by the lid 1 and the cup member I. diiference is established between the oil and water phases such that the difference in pressure across the membrane I3 is equal to the desired capillary pressure. At this time the whole assembly is immersed in an oil bath heated to reservoir temperature. While maintaining the aforesaid temperature and pressure the increase in the water level in the glass tube 36 is observed. By this observation data are obtained indicating the rate of water displacement from the rock fragment and also the total amount of water displaced over a period of time.

The operation as described above may be modifled to compensate for the effects of reservoir ross fluid pressure by initially establishing reservoir pressure conditions on both sides of the membrane i3 and then creating a pressure difference across the membrane equal to the capillary pressure.

The foregoing operations simulate what has occurred in nature by the intrusion of oil into a water bearing formation. The water displaced from the core fragment gives a measure of the amount of water which was displaced from the water bearing formation by the intrusion of oil. The water left in the fragment which can be determined by subtracting the displaced water from the known pore space of the fragment is a measure of what is known as connate water in the reservoir rock, or, in other words, that water which is retained by capillary forces in the rock after the reservoir has become filled with oil. It may be mentioned here that the rock sample may be removed from the unit shown after the water displacement is completed and subjected to a treatment for the measurement of the actual wa ter contained in it.

The apparatus described is useful in many additional studies of reservoir behavior. For example, it can be used to determine the productivity That is; a pressure mination also the membrane is saturated with oil and the system below the membrane is also filled with oil. i

When the object is to determine the productivity or the rate at which 011 is produced from the reservoir under water drive the cell is turned upside down which involves turning the fitting 3| through 180 so that the glass tube 36 will still be upright. With the parts in this position water is introduced through inlet 9 and awater pressure equivalent to that available for the water drive in the reservoir is built up. Then again the rate and amount of increase in level in the glass tube 36 is observed.

If productivity under gas drive is to be studied the apparatus is used in the position shown with gas instead of water being admitted through the inlet port 9. For best results in a study of this type the gas employed is that which is available in a reservoir.

Referring to Fig. 3, parts corresponding to those skilled in experimental work of this type; for

example, it is possible by employment of this technique to determine in a rapid manner the correlation between connate'water and other measurable properties of the rock. Once this trend is established, and assuming these other properties are more easily measured, it then becomes possible to approximate values for connate water by these simpler measurements of associated rock properties rather than being required to employ the complete procedure originally required to establish the trend. For instance, simple methods for determining permeability are available. By using the procedure outlined above for securing average'values of connate water for a plurality of samples and by plotting these average values against measured shown in Fig. I bear the same numerals. In this embodiment the sample chamber is made larger to accommodate a larger number of samples. The number of posts it is suitably increased and the bar M is replaced by a plate 38. In this embodi ment the bottom plate i0 is replaced by a tray 39 in which is a plate 40 of porous material, such as porous alumina, ceramic material, sintered glass, unglazed porcelain, porous metal produced by the powdered metal technique or the like. This plate extends slightly above the upper edge of the tray 39 and the junction between the two is sealed by a suitable cement ii i. One side of the tray has a radial passage it in which is arranged the tube 26 as in Fig.1.

It will be understood that the same type of membrane can be utilized in both embodiments of the invention shown, the choice depending upon the capillary pressure conditions to be imposed. In general, the ceramic type material is used when low pressure operations are contemplated.

In utilizing the embodiment shown in Fig. 3 various techniques are possible. The number of samples which may be handled at once is limited only by the design of the apparatus, it being possible to construct it for any desired number of samples. The samples may be all from the same location in the reservoir, they may be from different levels in the reservoir, or they may be from laterally spaced points in the reservoir at the same level or different levels. Where they are taken from different depths the capillary pressure to be used in their study as a group is the average of their individual capillary pressures. The selection of samples to be processed simultaneously is determined by the object of the study and is left to the discretion of the operator.

The procedure to be followed in the use of this embodiment is the same in all respects as the procedure described with reference to the embodiment shown in Fig. l. The data obtained by observing the displacement of fluid into glass tube 36 as various pressure conditions are imposed reflects on the average character of the group of cores taken as a whole. Proper interpretation of these data is entirely apparent to those permeability of the samples; it is usually possible to establish a trend or rglationship between the two values. Where a clear relationship is shown to exist, connate water in other similar samples may be approximated by resorting tothe simple permeability measurements and determining connate water by reference to the aforesaid trend.

In the foregoing, reference has been made at various points to capillary pressure. Capillary pressure may be defined as the difference in pressure which exists at the curved interface between immiscible fiuid phases. pressure is related both to the interfacial tension between the phases and is also related to the geometry of the porous system in which the the reservoir at a point 20' above the level atwhich water exists as the continuous phase, the

value for capillary pressure in the equation given above may be shown to be about 2 p. s. i., assuming a density difference of 0.2 gm./cm. between the reservoir oil and water phases. Thus, it is evident from this equation that in the reservoir the forces of gravity (HDg) are balanced by opposing capillary forces (Po) and it is the attainment of this condition of equilibrium in laboratory experiments which is one of the principal objects of this invention.

The apparatus heretofore described also lends itself readily to study of the fluid distribution in a reservoir undera gradually increasing capillary pressure caused by gradual recession of the water table. The technique to be followed in this study is to bring the apparatus to equilibrium with pressures on both sides of the membrane equal and then gradually to increase the pressure of the displacing fluid, meanwhile observing the amount of rock fluid displaced. This gradual increase in pressure of displacing fluid can-be made equivalent to that obtained over a period of several years until a maximum capillary pressure is attained.

Various changes may be made in the design and arrangement of parts of the apparatus illustrated and described without departing from the scope of the present invention. Moreover, several studies for which the apparatus is adapted This capillary in addition to those specifically described will be evident to those skilled in the art.

The nature and objects of the present invention having thus been set forth and a specific embodiment of the same given, what is claimed and desired to be secured by Letters Patent is:

1. An apparatus for the study of the distribution of immiscible fluids in a subterranean reservoir which comprises a metal cell capable of withstanding the pressure in said reservoir, a semipermeable membrane arranged in said cell, means for sealing of]? the under side of said membrane from direct fluid contact with the interior of said cell while exposing the upper surface of said membrane to direct fluid contact with the interior of said cell, means for maintaining a fluid pressure head against the under side of said membrane, means for securing at least one rock sample from said reservoir against the upper side of said membrane in capillary contact therewith, means for introducing a displacing fluid into the interior of said cell and means for measuring the fluid head against the under side of said membrane.

2. An apparatus according to claim 1 in which the membrane is a thin cellophane sheet supported by a metal screen.

3. An apparatus according to claim 1 in which the membrane is a porous ceramic plate.

4. An apparatus for studying the distribution of immiscible fluids. in a subterranean reservoir comprising a metal cup member, a metal lid of substantial depth for said cup member, means for clamping said lid to said cup member to form a cell of the assembly, fluid sealing means between said cup member of said lid, a semi-permeable membrane arranged in said cup member, means for sealing the under surface of said membrane from direct fluid contact with the interior of said cell, a passage connecting the under surface of said membrane to the exterior of said cell, means for maintaining a head of fluid connected with said passage, means for securing at least one rock sample from said reservoir against the upper surface of said membrane in capillary contact therewith and means for introducing a displacing fluid into the interior of said cell.

WALTER DEAN ROSE.

REFERENCES CITED The following references are of record in the flle of this patent:

UNITED STATES PATENTS Number Name Date 2,254,006 Exline Aug. 26, 1941 2,330,721 Leverett Sept. 28, 1943 2,345,935 Hassler Apr. 4, 1944 2,465,948 Welge Mar. 29, 1949 

